U.S. President Barack Obama and United Nations Secretary-General Ban Ki-moon on Saturday led a chorus of world dignitaries marking the death of Muhammad Ali.

The boxing titan, humanitarian and civil rights activist, hospitalized with a respiratory infection, died Friday at age 74.

Obama, in a statement, likened the firebrand Ali to Nelson Mandela and Martin Luther King, Jr., saying Ali "shook up the world, and the world is better for it." Obama also posted a picture of himself sitting beneath the iconic ring photo of Ali standing over a defeated Sonny Liston after retaining the heavyweight championship in 1965.

“Rest in peace, Champ,” Obama tweeted.

U.N. chief Ban issued a statement describing how the world body "benefited from the life and work of one of the past century's great humanitarians and advocates for understanding and peace."

Former president Bill Clinton, who awarded Ali the Presidential Citizens Medal before leaving office in 2001, paid tribute in another tweet: “Goodbye my friend. You were Great in so many ways.”

The former president also issued a statement with his wife, Democratic presidential contender Hillary Clinton, that sought to capture Ali's unparalleled athleticism.

“From the day he claimed the Olympic gold medal in 1960, boxing fans across the world knew they were seeing a blend of beauty and grace, speed and strength that may never be matched again,” he said.

Republican presidential candidate Donald Trump also tweeted a tribute to Ali: “A truly great champion and a wonderful guy. He will be missed by all!"

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Muhammad Ali: A Legend And A Cultural Icon Dead at 74

Anti-war controversy, social justice activism

Basketball great-turned-journalist Kareem Abdul-Jabbar remembered the controversial Ali as a friend and mentor whose "greatness was his ability to be different things to different people."

In a lengthy essay Saturday, Abdul-Jabbar remembered Ali as an "unparalleled champion, faster and smarter than any heavyweight." He also cited Ali for giving voice to anti-establishment, anti-war American youth of the 1960s, describing him as "a defiant voice against the Vietnam war... ."

"To the African American community," Abdul-Jabbar wrote, "he was a black man who faced overwhelming bigotry the way he faced every opponent in the ring: fearlessly."

Brazilian soccer icon Pele used social media to lament the death, noting that “the sporting universe has just suffered a big loss. ... The sadness is overwhelming.”

Argentina’s Diego Maradona wrote “the best of all time has left... How can I not feel this loss... Surely he left because he could no longer give us more happiness. My condolences to his family.”

More coverage planned

Muhammad Ali will appear on the cover of Sports Illustrated June 13 - his 40th time as the magazine's top story. The article will include a 1970 photo showing Ali training in Miami Beach for his return to the ring after a three-year ban for refusing induction into the U.S. armed services.

In explaining his editorial decision, SI executive Paul Fichtenbaum described Ali as "a singular force of athletics, humanitarianism and social equality unlike anyone in our history.”